A widely used type of linear voltage regulators is the low dropout (“LDO”) voltage regulator. Dropout voltage is the term used to describe the minimum voltage across a regulator that is required to maintain output voltage regulation. LDO voltage regulators are widely used in modern low voltage (battery) power management integrated circuits (“ICs”) since they maximize the utilization of the available input voltage and can operate with higher efficiency than other types of voltage regulators.
Typical applications usually require that the LDO voltage regulator start as quickly as possible upon enable. Recently, however, many IC customers are demanding LDO voltage regulators with so-called “soft-start” capability, by which it is meant that the regulator output is slowly ramped to the desired final regulated voltage upon enable. This is primarily done so as to limit inrush current at initialization. This demand has risen especially with the widespread use of the Universal Serial Bus (“USB”). The USB standard imposes a stringent limit on the amount of current the USB power bus can source. If, during any mode of operation an LDO voltage regulator has to pull its current directly from the USB bus during startup, a major inrush current can flow through the regulator which exceeds the maximum current the USB bus can handle. Such an inrush of current can easily cause the system to malfunction, or cause an undesirable reset. This is so even if the full load current of the LDO regulator during steady-state operation is less than the maximum current the USB bus can handle, because during start-up, large current transients can occur, thus causing overload of the bus.
FIG. 1 is a block diagram showing the way in which modern power management ICs connect LDO regulators to a USB bus. Each of the N bus lines has its own separate LDO regulator circuit, and each such regulator circuit is susceptible to large inrush currents upon enable.
One of the more popular and simpler prior art ways of achieving soft-start is by slowly ramping up the reference voltage from which the LDO regulator derives its output voltage upon enable. This can be achieved by using a resistor-capacitor (“RC”) circuit branch to slow down the rising of the voltage reference at enable, or by other means. An LDO voltage regulator includes an error amplifier. By applying a slowly rising reference to the error amplifier, any large signal response that could potentially cause a major inrush of current is reduced. This method, while successful in many cases, can still fail for certain start-up conditions in which a sudden switching of load current through the LDO voltage regulator may still be activated.